The present invention relates to a method of and an apparatus for producing a glass blank for optical glass elements such as a lens, a prism, etc. used in optical instruments.
Recently, a number of attempts have been made in which an optical lens is formed by a single molding process without a polishing process. Thus, at present, optical lenses are being mass produced by such molding process. To this end, a method is most efficient in which a glass blank in a molten state is poured onto a die so as to be subjected to pressure molding. However, in this method, it is difficult to control contraction of glass during its cooling. Thus, this method is not suitable for accurate molding of lenses. Therefore, a method is generally employed in which a glass blank is preliminarily worked to a specific shape so as to be supplied between dies and then, is heated and pressure molded as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 58-84134 (1983) and 60-200833 (1985). Meanwhile, Japanese Patent Laid-Open Publication Nos. 2-34525 (1990) and 2-14839 (1990) teach such preliminary working of the glass blank.
Hereinbelow, the above mentioned known methods of producing the glass blank are described with reference to FIGS. 11 to 14. FIGS. 11a and 11b show a known method of molding a glass lens. In this known method, a spherical glass blank shown in FIG. 11a is supplied between upper and lower dies 52 and 53 so as to be pressured molded therebetween into a molded lens 54 of FIG. 11b by pressing the upper die 52 towards the lower die 53 in a sleeve die 51. The upper die 52 is slidably movable in the sleeve die 51.
FIG. 12a shows steps of a known method of producing a glass blank, while FIG. 12b shows steps of a known method of producing a polished lens. The steps of FIG. 12a are substantially the same as those of FIG. 12b. Namely, inspection step (17) and molding step (18) of FIG. 12a are, respectively, replaced by deposition step (17) and inspection step (18) in FIG. 12b.
FIGS. 13a and 13b show a known method of molding a glass element. As shown in FIG. 13a, molten glass is poured from an discharge outlet 55 so as to be received by a molding die 56 disposed below the discharge outlet 55. When a predetermined amount of the molten glass has been poured into the molding die 56, the molding die 56 is lowered at a velocity larger than a flow velocity of the molten glass from the discharge outlet 55 such that the molten glass flowing downwardly from the discharge outlet 55 is separated from the molten glass poured into the molding die 56. Thus, the molten glass poured into the molding die 56 is cooled in the molding die 56 until at least a surface of the molten glass is set, whereby the glass element is obtained.
FIG. 14 shows a known method of molding a glass member. In this known method, molten glass flowing from the discharge outlet 55 is dropped by its own weight or is cut by engaging opposed blades 65a and 65b with each other as shown in FIGS. 15a and 15b such that a molten glass lump 57 falls into a recess 60 of a molding die 58. At this time, gas such as air, inert gas or the like is blown into the recess 60 through a pore 59 opening into the recess 60 such that a gaseous layer is formed between the molten glass lump 57 and a side surface of the recess 60 of the molding die 58 as shown in FIG. 14. Thus, until at least a portion of a surface of the molten glass lump 57 reaches a temperature of not more than a softening point of glass, the molten glass lump 57 is held in the recess 60 substantially out of contact with the side surface of the recess 60 so as to be cooled such that the glass member is produced.
However, the glass lens molded by the above known method of FIGS. 11a and 11b has such a drawback that unless a maximum surface roughness of the glass blank supplied into the die is 0.1 .mu.m or less, the glass lens has defects on the optical face and thus, cannot fulfil optical performances sufficiently.
Meanwhile, the known method of producing the glass blank shown in FIG. 12a requires time and cost equivalent to those for working of a glass lens, so that the glass blank becomes expensive.
Furthermore, in the known method of molding the glass element shown in FIGS. 13a and 13b, when the molten glass is brought into contact with the molding die 56, the molten glass is cooled rapidly. As a result, large wrinkled defects are produced on a contact surface of the molten glass relative to the molding die due to thermal shrinkage. Therefore, in the case where an optical element is formed by this known method, defects are produced on the optical face of the optical element undesirably.
Moreover, in the known method of molding the glass member of FIG. 14, not only it is difficult to deform the glass member into a desired shape but amount of the molten glass flowing from the discharge outlet is not stable due to influence of the gas blown into the recess through the pore of the molding die, which recess is disposed immediately below the discharge outlet.